From Bookwalker:
His girlfriend is so gorgeous that every guy turns to look at her. But this relationship is fake. Then why does she look so happy to be with him, an average guy who is not good at anything? Seiji entered Souhou Academy, where the wealthiest families send their children, after he passed the transfer exam. A new student was rare, so he had been spending his high school days in solitude until the day a girl asked him to be a fake boyfriend of her sister. If she doesn't have one, their mother will go find a fiancé, the girl says. Her sister, Sara, was a cool beauty from a wealthy family, admired by everyone in the Academy. Seiji couldn't understand why he was chosen but accepted the offer. From that day, he started to spend time with her. Although it was supposed to be a fake relationship, Sara looked like she is actually enjoying being with him. Seiji became confused about what Sara wants from him.